Sensor for an electrochemical detecting element

ABSTRACT

A sensor for an electrochemical measuring probe for determining the concentration of a gas component in a measuring gas, in particular for determining the oxygen concentration in the exhaust gas of internal combustion engines. The sensor includes a Nernst cell mounted on one side of a support on its surface, and an electric heater situated on the other side of the support. To avoid a bimetallic effect when the heater is rapidly heated up and the associated high tensile stresses in the longitudinal edges of the support, the heater is situated on a second support, which is attached to the first support, is made of the same material, and is at least approximately the same thickness as the first support.

FIELD OF THE INVENTION

The present invention is directed to a sensor for an electrochemicalmeasuring probe for determining the oxygen concentration in gases, inparticular in the exhaust gas of internal combustion engines.

BACKGROUND INFORMATION

In a conventional sensor of this type, as described in, e.g., GermanPatent Application No. DE 197 51 128, a heater designed as a wave-shapedelectrical resistor is printed onto the surface of a support layerfacing away from a Nernst cell and is covered by a likewise printedcover layer made of aluminum oxide (Al₂O₃). The cover layer and heaterare co-fired jointly with the support layer. A porous adhesive layer issintered onto the surface of the support layer receiving the Nernstcell, and a gas-tight base layer made of yttrium-stabilized zirconiumoxide (ZrO₂) is printed onto the porous adhesive layer. The referenceelectrode and its lead, as well as a sacrificial layer providing thereference channel, are then printed in successive printing steps. Ionconductors, the solid electrolyte, and the external electrode with itslead are then printed on. An external porous protective layer is printedonto the external electrode and a gas-tight cover layer is printed ontothe lead to the external electrode.

SUMMARY

An example sensor according to the present invention may have theadvantage that the heater is located in the middle of the sensor andgenerates a uniform low tensile stress on each side of the sensor. Abimetallic effect occurring in the conventional sensor when it is heatedup rapidly and the resulting high tensile stresses in the longitudinaledges of the support are prevented. When only two ceramic foils areneeded for the two supports, which may be made either ofyttrium-stabilized zirconium oxide (ZrO₂) or of aluminum oxide (Al₂O₃),the layouts for both the heater and the Nernst cell may be manufacturedgeometrically completely independently of one another. Considerably lesspositional accuracy is needed in this case. The sensor has an excellentquick-start response, because only a low heat capacity must be heatedup, and the central positioning of the heater allows high heat-up ramps.The example sensor design according to the present invention featuringtwo separate supports for the heater and the Nernst cell allows for asecond measuring cell to be mounted on the surface of the second supportfacing away from the Nernst cell. This measuring cell may be eitheranother Nernst cell or a cell having a different sensitivity, e.g., forhydrocarbons.

An example sensor according to the present invention may have theadvantage that, due to the porous filling of the reference channel, thelatter does not collapse when the sensor is pressed into a sensorhousing, even in the case of a thin cover layer. Each electrode isprovided in a simple manner with a double lead having a low ohmicresistance. The external electrode and reference electrode are onlyseparated by a printed layer, namely the solid electrolyte, and havetherefore nearly the same temperature. Due to the bottom lead insulationon the first support, which may also cover the area of the subsequentlyprinted-on reference channel, the Nernst cell may be insulated againstinterference from the heater. In this case, the first support is incontact with the material of the probe housing.

An example method according to the present invention for manufacturingthe above-described sensor may have the advantage that it is simple andcost-effective to carry out and permits the manufacture of a sensorhaving a low installation height.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is explained in detail with reference to theexemplary embodiments illustrated in the figures and the descriptionbelow.

FIG. 1 schematically shows a cross section of a sensor for anelectrochemical measuring probe near its measuring gas side endaccording to section line I-I in FIG. 4.

FIG. 2 schematically shows a cross-section of the sensor near its endaway from the measuring gas according to section line II-II in FIG. 4.

FIG. 3 schematically shows a top view of the individual functionallayers of the sensor illustrated in FIG. 1, without the heater.

FIG. 4 schematically shows a top view of the four successive bottomlayers illustrated in FIG. 3.

FIG. 5 schematically shows an illustration of a modified sensor similarto FIG. 1.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

In accordance with an example embodiment of the present invention, thesensor for an electrochemical measuring probe for determining the oxygenconcentration in the exhaust gas of internal combustion engines,illustrated in FIGS. 1 and 2 in two different section views, also knownas planar lambda-1 probe or planar Sprung probe, has a first support 11made of yttrium-stabilized zirconium oxide (ZrO₂), on which a Nernstcell 12 is mounted, and a second support 13 made of yttrium-stabilizedzirconium oxide, on which an electric heater 14 is mounted. The twosupports 11, 13 have the same thickness.

Heater 14 includes a wave-shaped flat resistor 15, which is embedded inan aluminum oxide (Al₂O₃) insulator 16 and is connectable to a heatingvoltage. Resistor 15 and insulator 16 are printed, for example, on thesurface of second support 13. Insulator 16 is advantageously enclosed bya sealing frame 17 made of a solid electrolyte 21. First support 11 ispermanently bonded to insulator 16, e.g., via an insulating andnon-ion-conducting foil binder. Alternatively, both supports 11, 13 maybe made of aluminum oxide (Al₂O₃). This does not require an insulator16, and the aluminum oxide sealing frame encloses resistor 15.

Nernst cell 12 has a reference electrode 18 on the measuring gas sideend of support 11, which is exposed to a reference gas, normally air,via a reference gas channel 19, and an external electrode 20 exposed tothe measuring gas, i.e., the exhaust gas. Reference electrode 18 andexternal electrode 20 are situated on either side of solid electrolyte21 facing away from one another and are provided with electrical leadsformed by flat conductor tracks. Reference gas channel 19 is porouslyfilled with aluminum oxide, for example, and runs in the middle betweentwo pairs of leads lying directly on top of one another. One pair madeup of a first lead 22 and second lead 23 belongs to reference electrode18, and one pair made up of a first lead 24 and a second lead 25 belongsto external electrode 20. First lead 22 of reference electrode 18 andfirst lead 24 of external electrode 20 are situated in the plane ofreference electrode 18, first lead 22 being connected to referenceelectrode 18 to form a single piece. Second lead 23 of referenceelectrode 18 and second lead 25 of external electrode 20 are in theplane of external electrode 20, second lead 25 being connected toexternal electrode 20 to form a single piece. Each pair of leads 22, 23and 24, 25, respectively, is covered by a bottom insulation layer 26 anda top insulation layer 27, bottom insulation layer 26 being situateddirectly on the surface of first support 11 and being cut out in thearea of reference gas channel 19, while top insulation layer 27 coverssecond leads 22, 23 and reference gas channel 19 between them. Leads 22,23 and 24, 25, respectively, which lie directly on top of one another ineach pair, form, at the end of first support layer 11 away from themeasuring gas, terminal contacts 28, 29 having a larger cross section.As FIG. 2 shows, reference electrode 18 formed on the measuring gas sideend of the sensor lies directly on top of filled porous reference gaschannel 19. Solid electrolyte 21 is situated between reference electrode18 and external electrode 20, and external electrode 20 is covered by agas-permeable protective layer 30. FIG. 3 shows a top view of theabove-described individual functional layers of Nernst cell 12. Thesefunctional layers are situated on top of one another starting with firstsupport 11. Support 11 is—like support 13—designed as a ceramic foil,onto which the other functional layers are printed.

The sensor thus described is manufactured as follows, reference beingmade to FIG. 3 and the reference numerals provided there:

Bottom insulation layer 26, 27 is printed onto support foil 11, it beingcut out in the area of reference gas channel 19. Alternatively,insulation layer 26 may also cover the area of reference gas channel 19.Filled, porous reference gas channel 19 is subsequently printed, itbeing preferably manufactured of open porous aluminum oxide (Al₂O₃) .Reference electrode 18, its first lead 22, and second lead 23 areprinted for external electrode 20 as the next functional layer, and endcontacts 28, 29 are formed. Solid electrolyte 21 made of yttriumoxide-stabilized (Y₂O₃) zirconium oxide (ZrO₂) is then printed inseveral thin printed layers. External electrode 20 and its second lead25, which is congruent to first lead 24, follows, and at the same timesecond lead 23 for reference electrode 18, which is congruent to firstlead 22, is also printed. Top insulation layer 27, which covers secondleads 23, 25 and reference gas channel 19, is subsequently printed. Theopen porous aluminum oxide of reference gas channel 19 ensures optimumconnection to above-lying insulation layer 27, which has closed poresand is also made of aluminum oxide. Finally, gas-permeable protectivelayer 30 is printed onto external electrode 20.

FIG. 4 shows how bottom functional layers 11, 26, 19, and 18 togetherwith 22 and 24 in FIG. 3 are situated on top of one another. Theremaining four functional layers 20 including 25, and 23, 27, and 30 inFIG. 3 are printed on top of one another in the geometry shown,resulting in the sensor illustrated as a cross section in FIGS. 1 and 2.The individual functional layers are preferably printed using the screenprinting method.

The described design of the sensor in FIGS. 1 and 2 having heater 14situated in the middle of the sensor permits a second Nernst cell 12′ tobe mounted on the surface of second support 13 facing away from firstsupport 11, as FIG. 5 shows as a cross section. The design of Nernstcell 12′ corresponds to that of previously described Nernst cell 12, sothat the same components are provided with the same reference numerals.Instead of a Nernst cell 12′, a cell having a different sensitivity, forexample for hydrocarbons, may also be provided.

1-13. (canceled)
 14. A sensor for an electrochemical measuring probe fordetermining a concentration of a gas component in a measuring gas,comprising: a first support; a Nernst cell mounted on a surface of oneside of the first support, the Nernst cell including a referenceelectrode exposed to a reference gas, an external electrode exposed to ameasuring gas, and an ion-conducting solid electrolyte separating thereference electrode from the external electrode; an electric heatersituated on another side of the first support; and a second supportaffixed to the first support, the heater being attached to the secondsupport, the second support being made of a same material as the firstsupport and having at least approximately a same thickness as the firstsupport.
 15. The sensor as recited in claim 14, wherein the sensor isfor determining an oxygen concentration in exhaust gas of an internalcombustion engine.
 16. The sensor as recited in claim 14, wherein thefirst support and the second support are made of yttriumoxide-stabilized (Y₂O₃) zirconium oxide (ZrO₂), and wherein the heateris embedded in an insulator.
 17. The sensor as recited in claim 14,wherein the first support and the second support are made of aluminumoxide (Al₂O₃).
 18. The sensor as recited in claim 14, furthercomprising: one of a second Nernst cell or a measuring cell having adifferent measuring sensitivity, the one of the Nernst cell or themeasuring cell being mounted on a surface of the second support facingaway from the heater.
 19. The sensor as recited in claim 18, wherein themeasuring cell is configured to meausre hydrocarbon.
 20. The sensor asrecited in claim 14, wherein the first support and the second supportare foils.
 21. The sensor as recited in claim 14, wherein the referenceelectrode is situated in a porously filled reference channel, which runsbetween two pairs of leads lying on top of one another, of which onepair of the leads belongs to the reference electrode and one pair of theleads belongs to the external electrode, a bottom lead of each pair ofleads is in one plane with the reference electrode and a top lead is inone plane with the external electrode, and each pair of leads is coveredby a bottom and a top insulation layer.
 22. The sensor as recited inclaim 21, wherein the external electrode is covered by a gas-permeable,porous protective layer.
 23. The sensor as recited in claim 22, whereinthe protective layer is made of aluminum oxide.
 24. The sensor asrecited in claim 21, wherein the reference channel is made of porousaluminum oxide.
 25. The sensor as recited in claim 21, wherein the leadsare flat conductor tracks.
 26. The sensor as recited in claim 21,wherein individual layers, electrodes, and leads are printed one on topof the other on the first support.
 27. A method for manufacturing asensor, comprising: printing a bottom insulation layer onto a firstsupport; subsequently printing a porously filled reference gas channelonto the first support in a middle of an insulation layer and protrudingbeyond a bottom insulation layer on a measuring gas side end;subsequently printing a reference electrode and a first lead of thereference electrode and a first lead of an external electrode in such away that the reference electrode covers a measuring gas side frontsection of the reference gas channel and the first lead of the referenceelectrode and the first lead of the external electrode rest on thebottom insulation layer; printing a solid electrolyte in an area of thereference electrode in several thin printed layers; printing in a jointprinting operation, the external electrode onto the solid electrolyte, asecond lead of the external electrode onto the first lead of theexternal electrode, and a second lead of the reference electrode ontothe first lead of the reference electrode; printing a top insulationlayer on the first and second leads of the external electrode and thereference gas channel between them; and printing a protective layer onthe external electrode.
 28. The method as recited in claim 27, whereinprinting is performed using a screen printing method.